This invention relates to a streak tube, which may be utilized, for example, in analyzing a light source whose strength changes rapidly.
The time resolving power of the streak tube is excellent, and it may be used to indicate a change of approximately one nanosecond in light incident thereon on a phosphor screen within a length of tens of milimeters, and to read out the change in less than two picoseconds. Thus the streak tube has conventionally been used to analyze the waveforms analysis of laser pulse light beams, etc.
First, the structure of the conventional streak tube and the problem to be solved according to this invention will be explained briefly with reference to the attached FIG. 1.
FIG. 1 is a longitudinal section showing the structure of a conventional streak tube. A schematic diagram showing a relation between a photocathode surface of the tube and an optical image is also shown in the figure.
One end of a vacuum and air-sealed tube 3 forms a window 1 through which an optical image to be analyzed is received, and the other end of the tube 3 forms a window 2 from which the processed image of the optical image is emitted. Between the windows 1 and 2 and along the tube axis of the tube 3 there are provided a photocathode surface 4, a mesh electrode 5, a focusing electrode 6, an aperture electrode 7, a deflection electrode 8 and a phosphor screen 9. The voltage applied to the mesh electrode 5 is higher than the voltage on the photocathode surface 4, and the voltage applied to the aperture electrode 7. Also, the same potential as that applied to the aperture electrode 7 is applied beforehand to the phosphor screen 9. Suppose that a line optical image 4a is projected onto the photocathode surface 4 through the window 1 by using an optical device, not shown, and that the image 4a passes through the center of the photocathode surface 4. The photocathode surface 4 emits an electron image which corresponds to the optical image projected thereon, and the emitted electrons are then accelerated by the mesh electrode 5 and converged by the focusing electrode 6. The electrons then pass the aperture electrode 7 and move towards the phosphor screen 9 through a gap in the deflection electrode 8. A deflection voltage is applied to the deflection electrode 8 for the period of time during which the line electron image passes the gap of the deflection electrode 8. The direction of the electric field generated by the deflection voltage is perpendicular to the axis of the tube 3 and also to the line electron image (that is, perpendicular to the plane of the paper in the sectional view of FIG. 1). The strength thereof is in proportion to the deflection voltage. On the phosphor screen 9 the line electron beam is scaanned in the perpendicular direction with respect to its line direction. Therefore the line image at first projected on the photocathode surface 4 is finally formed on the phosphor screen 9 as an optical image arranged one after one in the time sense in the perpendicular direction to the original line direction, the final image on the phosphor screen being called a streak image. The change in brightness of the arranged direction, or sweeping direction of the streak image thus indicates the time sense change of the strength of the incident optical image. Several methods hve conventionally been used to quantitate the change of time of the strength of the incident light from the streak image obtained on the phosphor screen 9 of the stream tube. One of them is a method of recording the streak image on a film and measuring its blackening density. Another method is to pick up a streak image by a TV camera and to then analyze the video signal thus obtained. Generally speaking, the light to be measured by the streak tube is extremely weak and moreover the time for measurement is extremely short. Therefore a good S/N ratio cannot be obtained. In order to improve this ratio, in either of the above methods the strength of the streak tube which corresponds to a certain time is either integrated or added. For example, in the former method, an aperture in the form of a slit is used for the film to seek an average blackening density of part of the film. In the latter method, the streak image on the phosphor screen is picked up by a TV camera so as to make the sweeping direction of the streak tube match with the vertical scanning direction thereof and a video signal is integrated for each scanning line. In order to employ these methods, the optical image of the same time is preferably a straight line image on the phosphor screen. However, as will be fully explained later, such an optical image does not become a straight line on the phosphor screen. Even if the optical image of the same time is a curve, a curved slit aperture may be used if this is known beforehand according to the above first method. According to the second method, the brightness signal of the optical image representing the same time is extracted from a video signal and it may be integrated. However, the degree of curveature in fact varies due to the change of sweeping speed, and so the form of slit must be changed or a further complicated operation becomes necessary. Calculation for such complicated operation requires, for example, several seconds even when a computer is used, and thus the operation can not follow the frame period of a video signal which is of one several tenth second. Also it cannot follow incident light which repeats at intervals of less than several seconds. On the other hand, the electron optics system of the streak tube other than the deflection system is revolutionally symmetrical with its axis running from the center of the photocathode surface to the center of the phosphor screen. And the electron image emitted from the photocathode surface is focused on an axis between the photocathode surface and the phosphor screen with an electron lens of the above electron optics system. The image is then reversed with respect to both the vertical and axial directions and projected thereafter onto the phosphor screen. The above is further explained hereinafter with reference to FIG. 1. Electrons emitted from the center a of the photocathode surface collide against the center b of the phosphor screen 9. Electrons from the point a' which is apart from the center a of the photocathode surface 4 pass through a focusing point 11 and collide against a point b' which is likewise apart from the center b of the phosphor screen 9. Electrons emitted from the photocathode surface are very rapidly accelerated in the direction of the tube axis between the photocathode surface 4 and the aperture electrode 7 and pass the aperture electrode 7 with a constant speed. However, they are not accelerated in the direction of the tube axis between the aperture electrode 7 and the phosphor screen 9, but are only deflected in the direction perpendicular to the plane of the figure by a deflection electric field. Therefore, the vertical (to the plane of the paper) position of the electrons passing through the center a of the photocathode surface 4 and emitted from any point on the line perpendicular to the deflection field (that is the line through a and a' of FIG. 1) at which the electrons collide with the phosphor screen 9 depends solely on the condition of the deflection voltage applied to the deflection electrode 8. Therefore if electrons enter into the deflection field simultaneously, the colliding position which is in the perpendicular direction to the sheet must the same in height. However, the distance from the point a' which is apart from the center a of the photocathode surface 4 to the incident point in the deflection field is longer than the distance from the center a. Therefore when the electrons are emitted simultaneously from the center a of the photocathode surface 4 and from the point a' apart from the center a, the electrons emitted from the latter point enter into the deflection field later than those emitted from the center a. In other words, a straight line optical image 4a entered from the photocathode surface 4 becomes a curve 12 which is convex in the sweeping direction. This is schematically shown in FIG. 2. The arrow A in the figure indicates the sweeping direction. When the sweeping speed is changed, the curvature of the curve 12 will change.
As above explained, if a line optical image which enters into the photocathode surface 4 at the same time appears as a curve on the phosphor screen 9, it is not proper to use the method of picking up the phosphor screen by a pick-up tube and of integrating the video signal of each scanning line. This is because a signal obtained by integration is only a mixed image of different time.
The object of this invention is therefore to provide a streak tube of the type having an electronic optical system where an electron image focused and then formed as an image, in which a linear image entered into the photocathode surface at the same time will appear as a linear image on the phosphor screen irrespective of the sweeping speed.